A swing tag is an informational text attached to a garment for sale and can be also known as a swing ticket.
Unlike a laundry symbol or size marker that are sewn into a garment, swing tags are not part of a garment: a swing tag may be attached to the garment by thread or by a plastic joiner.
The term swing tags in the UK is used in a similar way to "hang tags" in America.
They are the small pieces of card that denote price, size, barcodes and other information on garments and many retail products.
[1]